The disclosed embodiments relate generally to methods and systems for isolating a medium voltage.
Electrically alternating current (AC) power is generally available at several different standardized voltage levels. Levels up to approximately 600 volts may be classified as low voltage (LV). Levels above approximately 69,000 volts may be classified as transmission voltages. Levels between LV and transmission voltages may be classified as medium voltage (MV).
Electrical equipment of a high power rating may be fed from MV power. MV power presents hazards of electrocution and flash burns. Therefore, safety codes generally require that access to MV power be restricted to trained service personnel. In order to restrict the access to MV power, portions of equipment containing MV circuits may be enclosed in a metal compartment or located in a restricted room or vault. As used herein, a compartment, room, vault, or other structure that physically separates some or all components of an MV circuit from non-MV components are referred to as a medium voltage compartment. The portions of the equipment containing MV circuits may be considered to be on the MV side of the equipment, whereas the portions of the equipment only containing LV circuits, and therefore having less restricted access, may be considered to be on the LV side of the equipment.
Electrical equipment fed by MV power may also contain LV devices for protection or control. LV devices may include, but are not limited to, thermostats. The LV devices may be wired into LV circuits which may include interface devices that can be touched by a human operator. Interface devices may include, but are not limited to, switches, pilot lights, meters, display screens, etc.
Safety codes generally require that protective means be provided to prevent the MV power from invading the LV circuits, even during an arcing fault in the MV circuits. Such protective means may include separating the LV wiring from the MV wiring by a metal barrier with a specified minimum thickness. At the specified minimum thickness, the metal barrier is able to resist being melted by plasma or radiation from an MV arcing fault for a time interval long enough that the fault will first be cleared by MV protective devices such as, for example, fuses, circuit breakers, etc.
FIG. 1 illustrates a simplified representation of a prior art apparatus 100 (e.g., electrical equipment of a high power rating) which includes at least one MV circuit 161 including MV wiring and other MV components, and at least one LV circuit 150 including LV wiring and other LV components. The MV circuit 161 is contained within a MV compartment 110 located on a MV side of the apparatus, and the LV circuit 150 is contained within both the MV compartment 110 and a LV compartment 130. The MV compartment includes a grounded metal wall 112 which functions to isolate the MV compartment from the LV compartment. The MV compartment 110 may also contain one or more MV devices 162.
One of the LV circuits 150 includes a plurality of series-connected normally-closed LV thermostats 131-134 which are installed in the MV compartment 110, and a LV relay 136 (e.g., over-temperature relay) which is installed in the LV compartment 130 and is connected in series with the LV thermostats 131-134. The LV thermostats 131-134 are utilized to monitor the temperatures of critical components in the MV compartment 110, and the LV relay is utilized to open or close one or more LV control circuits in the LV compartment 130.
In operation, 120 VAC control power 140 from the LV compartment is applied through the normally-closed LV thermostats 131-134 to the LV relay 136, thereby energizing the LV relay 136 and moving the contacts of the LV relay 136 to a closed position which closes a control circuit 138 in the LV compartment. If any of the LV thermostats 131-134 detects an excessive temperature, the given LV thermostat opens, thereby de-energizing the LV relay 136 and moving the contacts of the LV relay 136 to an open position which opens the LV control circuit 138 in the LV compartment 130. The opening of the LV control circuit 138 causes an alarm 142 signal to be generated. In response to the alarm signal, or in the alternative, a warning message may be displayed, the power may be interrupted, etc.
As shown in FIG. 1, the LV wiring 150, which carries the 120 VAC control power, passes through the grounded metal wall 112 of the MV compartment. When an arcing fault in a MV circuit (for example, 161) occurs, plasma 160 resulting from the arcing fault may contact the LV circuit which includes the LV thermostats 131-134 and the LV wiring 150 in the MV compartment 110. When the plasma 160 contacts the LV thermostats 131-134 or the LV wiring 150, the high temperature and/or high voltage of the plasma 160 may cause the insulation of the LV thermostats 131-134 and/or LV wiring 150 to fail. The failure of the insulation may create a direct connection 170 between the MV circuit 161 and the LV circuit 150 via the plasma 160, thereby applying MV to the LV thermostats 131-134, the LV wiring, and to other LV components connected thereto. As the devices and wiring in such LV circuits are generally not sufficiently insulated to withstand the far greater MV, their insulation may also break down at locations not directly exposed to the plasma. The MV may continue to jump from one LV circuit to another LV circuit in the above-described manner until the MV reaches a human interface device 180 and creates a potentially lethal shock hazard.
To minimize the risk associated with potential arcing faults, each LV device located in the MV compartment 110 may be enclosed in a grounded metal box, and all LV wiring located in the MV compartment 110 may be run in grounded metal conduit. For such implementations, the metal in the grounded metal boxes and in the conduit would be of a thickness sufficient to resist being melted by plasma or radiation of the MV arcing fault for a desired time interval. However, such configurations tend to be difficult and expensive to implement, especially so for applications having numerous LV devices located in the MV compartment and/or LV devices in scattered locations in the MV compartment.